Optical metrology tool equipped with modulated illumination sources

ABSTRACT

The system includes a modulatable illumination source configured to illuminate a surface of a sample disposed on a sample stage, a detector configured to detect illumination emanating from a surface of the sample, illumination optics configured to direct illumination from the modulatable illumination source to the surface of the sample, collection optics configured to direct illumination from the surface of the sample to the detector, and a modulation control system communicatively coupled to the modulatable illumination source, wherein the modulation control system is configured to modulate a drive current of the modulatable illumination source at a selected modulation frequency suitable for generating illumination having a selected coherence feature length. In addition, the present invention includes the time-sequential interleaving of outputs of multiple light sources to generate periodic pulse trains for use in multi-wavelength time-sequential optical metrology.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to and claims the benefit of theearliest available effective filing date(s) from the following listedapplication: The present application constitutes a continuationapplication of U.S. patent application Ser. No. 16/284,950, filed onFeb. 25, 2019, which constitutes a divisional application of U.S. patentapplication Ser. No. 15/217,549, filed Jul. 22, 2016, which constitutesa continuation application of U.S. patent application Ser. No.13/648,768, filed on Oct. 10, 2012, which constitutes a regular(non-provisional) patent application of U.S. Provisional PatentApplication Ser. No. 61/545,965, filed on Oct. 11, 2011, whereby each ofthe above-listed patent applications is incorporated herein by referencein their entirety.

TECHNICAL FIELD

The present invention generally relates to a method and system foroptical metrology, and, in particular, a method and system for opticalmetrology with time-modulated illumination sources.

BACKGROUND

As demand for ever-shrinking semiconductor device features continues toincrease so too will the demand for improved optical metrologicaltechniques. Optical metrology techniques may include critical dimension(CD) metrology, thin film thickness and composition metrology, andoverlay metrology. These optical metrology techniques may be carried oututilizing a variety of optical architectures includingscatterometry-based optical systems, reflectometry-based opticalsystems, ellipsometry-based optical systems, and spectrometry-basedoptical systems.

Typically, optical metrology systems utilize light sources operating ina constant-current or in a constant-light-output mode in order to ensureoptical stability of the system as well as keeping noise levels withintolerated limits.

In optical metrology settings where coherent light sources areimplemented, the production of coherent artifacts, such as interferencefringes resulting from duplicate images (i.e., “ghosts) and speckle, aresignificant concerns in the operation of the given optical metrologytool. Due to the large coherence length of laser-based illuminationsources, minimizing the impact of coherent artifacts can be challenging.Coherent artifacts manifest in optical metrology settings where thecoherence length, often 100 m or more, of the utilized illumination islarger than the distance between light reflecting surfaces of themetrology tool. Such reflecting surfaces may include lenses, beamsplitters, optical fibers and the like. In this scenario, a primary beamwill constructively interfere with illumination from a parasitic beam,leading to the production of ghost induced interference fringes. Theinterference contributions may grow to such a degree that they possessintensity values on the same order of magnitude of the primary beam,thereby severely hampering the usability of the given optical metrologytool.

In addition, some metrology applications require time-sequencingintensity control of multiple illumination sources emitting differentwavelengths of light. The prior art accomplishes time-sequencingintensity control utilizing various optical-mechanical and electro-opticdevices such as shutters, acousto-optic devices, Pockels cells, and thelike. The prior art uses of such devices to control the time-sequencingof multiple illumination sources may lead to reduced stability andrepeatability.

Therefore, it would be advantageous to cure the shortfalls of the priorart and provide a system and method for mitigating the effects ofcoherence artifacts and additional noise sources in an optical metrologysetting. In addition, it would be advantageous to produce a system andmethod providing an efficient means for time-sequencing ofmulti-wavelength illumination source outputs for multi-wavelengthoptical metrology applications.

SUMMARY

An optical metrology tool is disclosed. In one aspect, the opticalmetrology tool may include, but is not limited to, a modulatableillumination source configured to illuminate a surface of a sampledisposed on a sample stage; a set of illumination optics configured todirect illumination from the modulated illumination source to thesurface of the sample; a set of collection optics; a detector configuredto detect at least a portion of illumination emanating from a surface ofthe sample, wherein the set of collection optics is configured to directillumination from the surface of the sample to the detector; and amodulation control system communicatively coupled to the modulatableillumination source, wherein the modulation control system is configuredto modulate a drive current of the modulatable illumination source at aselected modulation frequency suitable for generating illuminationhaving a selected coherence feature.

In another aspect, the optical metrology tool may include, but is notlimited to, a first illumination source configured to generateillumination of a first wavelength; at least one additional illuminationsource configured to generate illumination of an additional wavelength,the additional wavelength different from the first wavelength, the firstillumination source and the at least one additional illumination sourceconfigured to illuminate a surface of a sample disposed on a samplestage; a set of illumination optics configured to direct illumination ofthe first wavelength and illumination of the at least one additionalwavelength from the first illumination source and the at least oneadditional illumination source to the surface of the sample; a set ofcollection optics; a detector configured to detect at least a portion ofillumination emanating from a surface of the sample, wherein the set ofcollection optics is configured to direct illumination emanating fromthe surface of the sample to the detector; and a modulation controlsystem communicatively coupled to the first illumination source and theat least one additional illumination source, wherein the modulationcontrol system is configured to modulate a drive current of the firstillumination source in order to generate a first illumination waveformof the first wavelength, wherein the modulation control system isconfigured to modulate a drive current of the at least one additionalillumination source in order to generate an additional illuminationwaveform of the additional wavelength, wherein pulses of the firstillumination waveform are interleaved in time with at least pulses ofthe additional illumination waveform, the first illumination waveformand the additional illumination waveform having a selected waveformfrequency.

In another aspect, the optical metrology tool may include, but is notlimited to, a first illumination source configured to generateillumination of a first wavelength; at least one additional illuminationsource configured to generate illumination of an additional wavelength,the additional wavelength different from the first wavelength, the firstillumination source and the at least one additional illumination sourceconfigured to illuminate a surface of a sample disposed on a samplestage; a set of illumination optics configured to direct illumination ofthe first wavelength and illumination of the at least one additionalwavelength from the first illumination source and the at least oneadditional illumination source to the surface of the sample; a set ofcollection optics; a detector configured to detect at least a portion ofillumination emanating from a surface of the sample, wherein the set ofcollection optics is configured to direct illumination emanating fromthe surface of the sample to the detector; a first illuminationswitching device optically coupled to the first illumination source,wherein the first illumination switching device is configured to controltransmitted intensity of the illumination of the first wavelength; atleast one additional illumination switching device optically coupled tothe at least one additional illumination source, wherein the at leastone additional illumination switching device is configured to controltransmitted intensity of the illumination of the additional wavelength;and an illumination control system communicatively coupled to the firstillumination switching device and the at least one additional switchingdevice, wherein the illumination control system is configured tomodulate transmitted intensity of the illumination of the firstwavelength and transmitted intensity of the illumination of theadditional wavelength by controlling one or more characteristics of theillumination switching device.

In another aspect, the optical metrology tool may include, but is notlimited to, a modulatable pumping source configured to generateillumination beam; a plasma cell, the plasma cell including a bulb forcontaining a volume of gas; a set of optical elements configured toshape the illumination beam and focus the illumination beam from themodulatable pumping source into the volume of gas in order to maintain aplasma within the volume of gas; a set of illumination optics configuredto direct the illumination beam from the plasma cell to the surface of asample; a set of collection optics; a detector configured to detect atleast a portion of illumination emanating from a surface of a sample,wherein the set of collection optics is configured to directillumination from the surface of the sample to the detector; a pumpcontrol system communicatively coupled to the modulatable pumpingsource, wherein the pump control system is configured to modulate adrive current of the modulatable pumping source at a selected modulationfrequency in order to produce time-varying characteristics within theplasma contained within the plasma cell.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not necessarily restrictive of the invention as claimed. Theaccompanying drawings, which are incorporated in and constitute a partof the specification, illustrate embodiments of the invention andtogether with the general description, serve to explain the principlesof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1 illustrates a high level block diagram view of an opticalmetrology tool with one or more modulated illumination sources, inaccordance with one embodiment of the present invention.

FIG. 2A illustrates a high level schematic view of a reflectometry-basedoptical metrology tool with one or more modulated illumination sources,in accordance with one embodiment of the present invention.

FIG. 2B illustrates a high level schematic view of a ellipsometry-basedoptical metrology tool with one or more modulated illumination sources,in accordance with one embodiment of the present invention.

FIG. 3 illustrates a conceptual view of intensity spectra with andwithout illumination source modulation, in accordance with oneembodiment of the present invention.

FIG. 4A illustrates a high level schematic view of an optical metrologysystem equipped with multiple illumination sources each of a differentwavelength, in accordance with one embodiment of the present invention.

FIG. 4B illustrates a conceptual view of an interleaved pulse trainoutput of multiple illumination sources each of a different wavelength,in accordance with one embodiment of the present invention.

FIG. 5 illustrates a high level schematic view of an optical metrologysystem equipped with multiple illumination sources each of a differentwavelength, whereby intensity is controlled via an intensity switchingdevice, in accordance with one embodiment of the present invention.

FIG. 6 illustrates a high level schematic view of an optical metrologytool equipped with a spectral monitoring device, in accordance with oneembodiment of the present invention.

FIG. 7A illustrates a high level block diagram of a laser-pumped plasmabased optical metrology tool with a modulated pump source, in accordancewith one embodiment of the present invention.

FIG. 7B illustrates a high level schematic view of a laser-pumped plasmabased optical metrology tool with a modulated pump source, in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Referring generally to FIGS. 1 through 7B, an optical metrology toolhaving time-modulated illumination source capabilities is described inaccordance with the present invention. The present disclosure isdirected toward systems and methods for performing optical metrologywith one or more time-modulated illumination sources. Thetime-modulation of illumination emanating from one or more illuminationsources of the metrology system of the present invention provides forimproved precision, accuracy, and metrology throughput. Theimplementation of illumination modulation provided by the presentinvention aids in the suppression of coherence artifacts, such as, butnot limited to, interference fringes, coherent noise, and speckle, inmeasured optical signals (e.g., angular-resolved reflectivities orellipsometric parameters, polarization-resolved reflectivities orellipsometric parameters, wavelength-resolved reflectivities orellipsometric parameters, and the like). In addition, the presentinvention is further directed to the time-modulation of illuminationoutput of one or more pumping sources of a light-sustained plasma lightsource. The modulation of pumping source illumination output providesfor a reduction of noise levels in output illumination of the sustainedplasma light source.

Those skilled in the art will recognize that coherence artifact controlis a common challenge in designing an optical metrology tool. Insettings where a given optical metrology tool includes one or morecoherent light sources (e.g., lasers), the ability to control thecoherent effects (e.g., speckle and interference fringes) associatedwith stray light and ghosts becomes increasingly difficult. For example,a given optical metrology tool (see FIGS. 2A and 2B) includes multipleoptical surfaces. These optical surfaces may include, but are notlimited to, beamsplitters, lenses, optical fibers, objective lenssurfaces, apodizers and the like. Coherent illumination in opticalmetrology tools often results in detrimental speckle, fringes, and othercoherent artifacts. These can contribute to measurement noise andinstabilities, leading to degradation in precision and accuracy of themeasurement.

For example, in a given optical system a beam propagating through aprimary path may interfere with a parasitic beam reflected off anoptical surface (e.g., mirror, beam splitter, and the like) of theoptical system. To illustrate the detrimental effects of primary beamand parasitic beam interference, the primary beam and the parasitic beamare characterized by intensities I₁ and I₂. The superposition of thesetwo beams provides a combined beam output as follows:

I=I ₁ +I ₂+2√{square root over (I ₁ I ₂)} cos Ø  (Eq. 1)

where Ø represents the relative phase between the primary beam and theparasitic beam from a reflective surface of the optical metrology tool.For illustrative purposes, in a scenario where I₁=1 and I₂=0.0025(consistent with a parasitic beam reflecting off a surface with 0.25%reflectivity), the interference term of Eq. 1 will have a magnitude of10% of the primary beam in instances where the primary and the parasiticwave interfere constructively. This level of interference contributionis unacceptable in most optical metrology tools.

In contrast, in settings where the primary beam and parasitic beam arenot coherent with each other, the interference term of Eq. 1 goes tozero and the ghost correction for the metrology tool will have amagnitude of 0.25% of the primary beam, which is significantly moremanageable than the case described above.

Those skilled in the art will recognize that a typical spectrum of alaser (e.g., laser based on semiconductor diode technology) includes asingle narrow spectral line or multiple narrow spectral lines. Suchlaser sources commonly have long coherence lengths. Due to theirwavelength stability and low noise, single-wavelength laser sources areutilized ubiquitously throughout metrology applications. Due to thelarge coherence lengths of single-wavelength lasers, often exceeding 100m, suppression of coherence artifacts during implementation in ametrology setting for the reasons set forth previously herein.

FIG. 1 illustrates a block diagram view of an optical metrology tool 100equipped with time-modulated illumination capabilities, in accordancewith one embodiment of the present invention. In one aspect of thepresent invention, the system 100 includes a modulated illuminationsource 102 configured to illuminate a surface of a sample 106 (e.g.,semiconductor wafer) disposed on a sample stage, a detector 110configured to detect light reflected from the surface of the sample 106,and an optical system, which acts to optically couple the modulatedillumination source 102 and the detector 110. The optical system mayinclude a set of illumination optics 104 (e.g., lenses, mirrors,filters, and the like) suitable for directing and/or focusing light fromthe illumination source 102 to the sample 106. The optical system mayfurther include a set of collection optics 108 (e.g., lenses, mirrors,filters, and the like) suitable for directing light reflected orscattered from the surface of the wafer 106 to the detector 110. In thismanner, light may emanate from the illumination source 102 and travelalong the illumination arm (via illumination optics 104) to the surfaceof the sample 106. Light reflected or scattered from the sample 106 maythen travel from the surface of the sample 106 to the detector 110 alongthe collection arm (via collection optics 108) of the system 100. Inanother aspect, the optical metrology system 100 includes a modulationcontrol system 112 configured to modulate a drive current of themodulatable illumination source 102 (e.g., laser) at a selectedmodulation frequency.

It is noted herein that the optical metrology system 100 of the presentinvention may be configured to carry out any form of optical metrologyknown in the art. For example, the optical metrology system 100 isconfigured to perform at least one of the following metrologymethodologies: critical dimension (CD) metrology, thin film (TF)thickness and composition metrology, and overlay metrology.

It is further noted herein that the optical metrology system 100 of thepresent invention is not limited to a particular optical configurationor optical metrology function. In some embodiments, the opticalmetrology system 100 of the present invention may be configured as areflectometry-based metrology system. For example, the optical metrologysystem 100 may include, but is not limited to, a beam profilereflectometer (e.g., narrow band beam profile reflectometer) operatingin angle-resolved mode, a spectroscopic reflectometer, and the like.Spectral and single-wavelength beam profile reflectometry are generallydescribed in U.S. Pat. No. 6,429,943, filed on Mar. 27, 2001, which isincorporated herein by reference in the entirety.

In other embodiments, the optical metrology system 100 of the presentinvention may be configured as a scatterometry-based metrology system.For example, the optical metrology system 100 may include, but is notlimited, to, a broadband scatterometer (e.g., broadband spectroscopicscatterometer) or a narrow band scatterometer.

In additional embodiments, the optical metrology of the presentinvention may be configured as an ellipsometry-based metrology system.For example, the optical metrology system 100 may include, but is notlimited to, a beam profile ellipsometer or a spectroscopic ellipsometer.An overview of ellipsometry of the principles of ellipsometry isprovided generally in Harland G. Tompkins and Eugene A. Irene, Handbookof Ellipsometry, 1st ed, William Andrew, Inc., 2005, which isincorporated herein by reference in the entirety. In addition, Muellermatrix ellipsometry is discussed in detail in P. S. Hauge, MuellerMatrix Ellipsometry with Imperfect Compensators”, J. of the Optical Soc.of Am. A 68(11), 1519-1528, 1978; R. M. A Azzam, A Simple FourierPhotopolarimeter with Rotating Polarizer and Analyzer for MeasuringJones and Mueller Matrices, Opt Comm 25(2), 137-140, 1978; which areincorporated herein by reference in their entirety. Further, the conceptof “complete” ellipsometry is discussed in M. L. Aleksandrov, et. al.“Methods and Apparatus for Complete Ellipsometry (review)”, J. Appl.Spectroscopy 44(6), 559-578, 1986, which is incorporated herein in itsentirety. Spectral ellipsometry is generally described in U.S. Pat. No.5,739,909, filed on Oct. 10, 1995, which is incorporated herein byreference in the entirety. Beam profile ellipsometry is generallydescribed in U.S. Pat. No. 6,429,943, filed on Mar. 27, 2001, which hasbeen incorporated previously herein in its entirety.

Referring now to FIG. 2A, the optical metrology system 100 of thepresent invention may be embodied as a reflectometry-metrology tool,such as tool 200. FIG. 2A illustrates a high-level schematic view of areflectometry-based metrology tool suitable for implementation in thepresent invention. The reflectometer 200 may include an illuminationsource 102, an optical system, and a detector 110. The optical systemmay include a set of illumination optics 104, a beam splitter 204, and aset of collection optics 108. In this regard, light may emanate from theillumination source 102 and travel via the illumination optics 104 andbeam splitter 204 to the surface of the sample 106 disposed on samplestage 202. Light reflected from the sample 106 may then travel from thesurface of the sample 106 to the detector 110 via the collection optics108. Applicant notes that the configuration illustrated in FIG. 2A isnot limiting and is provided merely for purposes of illustration. Asnoted previously, it is anticipated that numerous reflectometer-basedoptical configurations may be utilized within the scope of the presentinvention.

Referring now to FIG. 2B, the optical metrology system 100 of thepresent invention may be embodied as a scatterometry/ellipsometry basedmetrology tool, such as tool 250. FIG. 2B illustrates a high-levelschematic view of an ellipsometry-based metrology tool suitable forimplementation in the present invention. The scatterometer/ellipsometer250 may include an illumination source 102, an optical system, and adetector 110. The optical system may include a set of illuminationoptics 104, a polarizer 206, a set of collection optics 108, and ananalyzer 208. The illumination and collection optics may includemirrors, lenses, beamsplitters, compensators, and the like. In thisregard, light may emanate from the illumination source 102 and travelthrough polarizer 206 and illumination optics 104 to the surface of thesample 106 disposed on sample stage 202. Light scattered from the sample106 may then travel from the surface of the sample 106 to the detector110 via the collection optics 108 and through the analyzer 208.Applicant notes that the configuration illustrated in FIG. 2B is notlimiting and is provided merely for purposes of illustration. As notedpreviously, it is anticipated that numerous scatterometry- andellipsometry-based optical configurations may be utilized within thescope of the present invention.

In one aspect of the present invention, the modulation control system112 is configured to modulate a drive current of the modulatableillumination source 102 at a selected modulation frequency. In oneaspect, the selected modulation frequency may be suitable for generatingillumination having a selected coherence feature.

In one embodiment, the selected coherence feature may include, but isnot limited to, a selected fringe visibility curve. In this regard, theselected modulation frequency may be suitable for generatingillumination having a selected fringe visibility curve. In a furtherembodiment, the selected modulation frequency may be suitable forgenerating illumination having a fringe visibility curve suitable forachieving coherence artifacts below a selected tolerance level (e.g., alevel where coherence artifacts are small enough to allow for operationof the metrology tool 100). In another embodiment, the modulationfrequency is suitable for generating a fringe visibility curveconfigured to suppress generation of interference fringes having anintensity above a selected level (e.g., intensity of interferencefringes small enough to allow for operation of metrology tool 100). Inyet another embodiment, the modulation frequency is suitable forgenerating a fringe visibility curve having a set of intensity peakspositioned at distances different from a characteristic optical pathlength of the optical metrology tool 100. The characteristic opticalpath length of the optical metrology tool 100 may include a distancebetween a first reflecting surface of the optical metrology tool and asecond reflecting surface of the optical metrology tool. In a furtherembodiment, the modulation frequency is suitable for generatingillumination with a fringe visibility curve substantially different froma fringe visibility curve of the illumination source in an unmodulatedstate. As described previously herein, by altering the fringe visibilitycurve of the illumination emitted by the illumination source 102 to asufficient degree, the impact from coherence artifacts (e.g., speckleand interference fringes) may be eliminated or at least reduced.

In another embodiment, the selected modulation frequency may be suitablefor generating illumination having a coherence length below a selectedlength (i.e., coherence length less than the distance between opticalcomponents of the system 100). For example, the selected modulationfrequency may be suitable for generating illumination having a coherencelength below the coherence length of the illumination source 102 in anunmodulated state (i.e., the coherence length of the illumination sourceprior to modulation). By way of another example, the selected modulationfrequency may be suitable for generating illumination having a coherencelength below a characteristic optical length of the optical metrologytool 100. For instance, the selected modulation frequency may besuitable for generating illumination having a coherence length smallerthan a distance between a first reflecting surface of the opticalmetrology tool 100 and a second reflecting surface of the opticalmetrology tool 100. As described previously herein, by reducing thecoherence length of the illumination emitted by the illumination source102 below the distance between reflecting surface within the metrologytool 100, the impact from coherence artifacts (e.g., speckle andinterference fringes) may be eliminated or at least reduced.

In one embodiment of the present invention, the modulation controlsystem 112 may act to drive the current of one or more laser lightsources at a selected frequency. For example, the modulation controlsystem 112 may act to modulate the drive current of a laser light source(e.g., multi-longitudinal mode laser light source) in order to achieve amodified fringe visibility curve in the laser light output, whereby themodified fringe visibility curve of the laser light source is adequatefor reducing coherence artifacts within the optical metrology tool 100below a selected tolerance level. By way of another example, themodulation control system 112 may act to modulate the drive current of alaser light source in order to generate illumination having a coherencelength below a selected level.

FIG. 3 illustrates a conceptual view of intensity spectra 302 from alaser source without drive current modulation and intensity spectra 304from the laser source with drive current modulation. As shown in FIG. 3,in the case of D.C. current drive, the spectrum 302 associated with thelaser source includes multiple longitudinal modes of the laser cavity.The spectrum 304 illustrated in FIG. 3 represents a broad envelope ofthe individual spectral peaks of curve 302. In this regard, the fastmodulation of the drive current of the laser source results in abroadening and smoothing of the intensity spectrum. The alteration ofthe fringe visibility curve of the laser source aids in suppressing thecoherence artifacts (e.g., interference fringes) discussed previouslyherein. Further, it is noted herein that the optical surfaces of a givenoptical metrology tool (e.g., 100) may be relatively easily configuredso that they are separated by distances sufficient to render the impactof parasitic interference negligible when the illumination source 102 isin the modulated state, such as a state consistent with the intensityspectra 304. Applicant notes that the above description related tofringe visibility curve, coherence length and distance between opticalcomponents is presented merely for illustrative purposes and should notbe interpreted as limiting.

In a further embodiment, the modulation control system 112 may modulatethe drive current of the modulatable illumination source 102 at afrequency in the radio frequency (RF) range. It is further noted hereinthat the particular frequency at which the control system 112 drives themodulatable illumination source 102 may be selected by trial and error.For instance, the implemented modulation frequency may be a frequencythat acts to reduce the coherence length of the illumination from thesource 102 below a characteristic optical path length of the opticalmetrology system 100. For example, the characteristic optical pathlength of the optical metrology system 100 may include a distancebetween two or more reflecting surfaces of the optical metrology tool100. In another instance, it is recognized that neither the coherencelength nor the fringe visibility curve (as described above) needs to bemeasured in order to implement to modulation of the illumination source102. In this sense, the control system 112 may sweep the modulationfrequency of the control system 112 until a satisfactory detector 110output is achieved.

In a further aspect of the present invention, the modulation controlsystem 112 of the optical metrology tool 100 is equipped with one ormore processors (not shown) communicatively coupled to the modulatableillumination source 102 and configured to control the modulation of theillumination source 102. The modulation control system 112 is configuredto execute modulation control algorithm 118 stored as a set of programinstructions 116 on a carrier medium 114 (e.g., non-transitory storagemedium). The program instructions 116 are configured to cause the one ormore processors of the control system 112 to carry out one or more ofthe various steps described in the present disclosure.

It should be recognized that the various control steps associated withthe modulation control as described throughout the present disclosuremay be carried out by a single computer system or, alternatively, amultiple computer system. Moreover, different subsystems of the system100 may include a computer system suitable for carrying out at least aportion of the steps described above. Further, the one or more computersystems may be configured to perform any other step(s) of any of themethod embodiments described herein.

The modulation control system 112 may include, but is not limited to, apersonal computer system, mainframe computer system, workstation, imagecomputer, parallel processor, or any other device known in the art. Ingeneral, the term “computer system,” “computing system(s),” or “computercontrol system” may be broadly defined to encompass any device(s) havingone or more processors, which execute instructions from a memory medium.

Program instructions 116 implementing methods such as those describedherein may be transmitted over or stored on carrier medium 114. Thecarrier medium 114 may be a transmission medium such as a wire, cable,or wireless transmission link. The carrier medium may also include anon-transitory storage medium such as a read-only memory, a randomaccess memory, a magnetic or optical disk, or a magnetic tape.

In another embodiment, the control system 112 may be communicativelycoupled to the illumination source 102 or any other subsystem of system100 in any manner known in the art. For example, the modulation controlsystem 112 may be communicatively coupled to the various sub-systems ofsystem 100 via a wireline or wireless connection.

In another embodiment of the present invention, the modulatableillumination source 102 may include any narrowband illumination sourceknown in the art. In one embodiment, the illumination source 102 mayinclude, but is not limited to, one or more lasers. For instance, thelaser light source may include, but is not limited to, one or moresemiconductor lasers. In another example, the laser source may include,but is not limited to, a diode-pumped solid-state laser. In anotherexample, the laser source may include, but is not limited to, a supercontinuum laser. Further, a first source emitting illumination in afirst spectral range may be combined with a second source emittingillumination in a second spectral range.

In another aspect of the present invention, the detector 110 may includeany light detection system known in the art suitable for implementationin a reflectometer, scatterometer, spectrometer or ellipsometer setting.For example the detector 110 may include, but is not limited to, atleast one of a CCD array, a CMOS array, one-dimensional photodiodearray, a two-dimensional photodiode array and the like.

FIG. 4A illustrates a multi-source illumination source 102, inaccordance with an alternative embodiment of the present invention. Inone aspect, the multi-source illumination source 102 of the opticalmetrology tool 100 includes two or more single illumination sources,each single source having a different output wavelength. In one aspect,the present invention provides for stable intensity balance and controlof multiple illumination sources. Those skilled in the art willrecognize that generally ON/OFF switching of illumination sources, suchas lasers and LEDs, may result in reduced stability and lead to anincrease in noise. Applicants have found that instability and noiseproduction is limited in settings where periodic waveforms areimplemented. In this manner, periodic waveform operation acts tomaintain the average stable thermal, electrical, and optical propertiesof the illumination sources, thereby improving wavelength stability andnoise reduction.

In one aspect of the present invention, the modulatable illuminationsource 102 of the system 100 may include a first illumination source 402a configured to generate illumination of a first wavelength (λ₁), asecond illumination source 402 b configured to generate illumination ofa second wavelength (λ₂), and up to, and including, an “Nth”illumination source 402 c configured to generate illumination of an Nthwavelength (λ_(N)).

In an additional aspect of the present invention, the modulation controlsystem 112 is communicatively coupled to the first illumination source402 a, the second illumination source 402 b and up to and including theNth illumination source 402 c by any means known in the art (e.g.,wireline or wireless connection). In a further aspect, the modulationcontrol system 112 is configured to execute a multi-source controlalgorithm 120 suitable for controlling the waveform of illuminationoutput for each of the sources 402 a-402 c. The modulation controlsystem 112 (via control algorithm 120) is configured to modulate a drivecurrent of the first illumination source 402 a in order to generate afirst illumination waveform (e.g., step-wise waveform of a selectedfrequency) of the first wavelength. In addition, the modulation controlsystem 112 is configured to modulate a drive current of the secondillumination source 402 b in order to generate a second illuminationwaveform of the second wavelength. In this manner, the pulses of thefirst illumination waveform are interleaved in time with the pulses ofthe second illumination waveform, the first illumination waveform andthe second illumination waveform having a selected waveform frequency.It is further noted herein that the combined waveform may include anynumber of component waveforms. In this manner, the pulses of the firstillumination waveform are interleaved in time with the pulses of thesecond illumination waveform and pulses of up to and including the Nthwaveform. The interleaving of the various waveforms from the sources 402a-402 c allows for time-sequential metrology measurements at multiplewavelengths. Moreover, because the modulation of the illumination fromthe light sources 402 a-402 c is accomplished with drive currentmodulation the present invention obviates the need for variousoptical-mechanical components such as optical shutters, chopper wheels,and the like. As such, the embodiment illustrated in FIG. 4A providesfor a simplified approach to multi-wavelength intensity control in theoptical metrology tool 100.

In another embodiment, the multi-source based illumination source 102may include a plurality of wavelength combiners 404 a, 404 b, and 404 cconfigured to combine the beams 403 a, 403 b, and 403 c emanating fromthe illumination sources 402 a, 402 b, and 402 c respectively. In thisregard, the wavelength combiners 404 a-404 c may act to spatiallycombine the beams, allowing for the temporal interleaving of the sourcewaveforms carried out by the algorithm 120 executed by the modulationcontrol system 112. Following the temporal interleaving and spatialcombination of the waveforms into beam, the combined waveform output 408may be directed to the illumination optics 104 of the optical metrologytool 100. It is further noted that the illumination source 102 mayinclude additional optical elements, such as steering mirror 406.Applicant notes that the optical configuration depicted in FIG. 4A anddescribed above is not limiting and should be interpreted as merelyillustrative. It is recognized herein that multiple equivalent opticalconfigurations may be implemented in order to spatially combine andtemporally interleave the waveforms of source 402 a, source 402 b, andup to and including source 402 c. The spatial combination of multiplelaser beams into a single combined beam is generally described by Hillet al. in U.S. patent application Ser. No. 13/108,892, filed on May 16,2011, which is incorporated herein in its entirety.

In one embodiment, the modulation of the first, second, and up to andincluding the Nth illumination sources carried out by the modulationcontrol system 112 may include switching the drive current of alaser-based or LED-based source. Switching of the source drive currentin this manner may produce a step-wise (i.e., ON/OFF) or nearlystep-wise waveform pattern for the illumination outputs for each of theillumination sources 402 a-402 c. In this regard, the multi-sourceapproached depicted in FIG. 4A allows for channel selection and relativeintensity control in a “color”-sequential manner. For the purposes ofthe present disclosure, the term “color” is used to describe the primarywavelength (e.g., peak wavelength) of each source. Further, the term“color” should not be interpreted to apply to any particular portion ofthe electromagnetic spectrum. It is anticipated that the wavelength of agiven source may reside well outside the visible spectrum. For instance,the spectral range of the output of sources 402 a-402 c may include thevisible, UV, and IR spectral ranges.

FIG. 4B illustrates a conceptual view of a graph 450 of a set ofinterleaved waveforms from three illuminations sources of differentwavelength λ₁, λ₂, and λ₃. The pulse train 451 depicted in FIG. 4B isrepresentative of either the input drive current of the illuminationsource or the output intensity of the illumination source for eachwavelength (e.g., λ₁, λ₂, and λ_(N)). In this regard, the pulse train451 may consist of a set of pulses 452 of wavelength λ₁, a set of pulses454 of wavelength λ₂, and up to and including a set of pulses ofwavelength λ_(N). It is noted herein that the input drive current (notshown in FIG. 4B), the duty cycle (i.e., width of each pulse for a givenwavelength), and output power (i.e., height of each pulse for a givenwavelength in FIG. 4B) is in general different for each wavelengthwaveform and is selected based on the requirements of the given opticalmetrology system. It is further recognized herein that the drive currentmay be switched between zero and the nominal peak current or,alternatively, may follow a more complex periodic scheme (e.g., thelower bound may be chosen to be a non-zero current). The frequency, dutycycles, and peak current and power levels of the waveform can be chosenfor optimal performance of the illumination sources (e.g., lasers) andother components of the metrology tool 100, such as a beam monitor, thedetector (e.g., one or more CCDs), and auto-focus subsystem, and thelike. It is further noted that changing the duty cycle and input currentmay also aid in achieving desired intensity levels and balance for themultiple light sources 402 a-402 c. It is further recognized that therepetition frequencies of the waveforms of the pulse train 451 may be onthe order of 100 Hz. As such, the multi-source repetition frequencies ofthe present invention are much slower than the single-source modulationfrequencies (e.g., RF frequencies) of the illumination source 102 asdiscussed previously herein. Therefore, control schemes for interleavedcolor-sequential operation (e.g., 100 Hz frequency range) and fornoise/coherence effects reduction (e.g., RF frequencies) may beimplemented simultaneously. In this regard, the control system 112 maydrive a given illumination source (e.g., 402 a-402 c) with multipleperiodic waveforms operating at significantly different timescales. Forexample, in addition to the interleaving of waveforms of source 402 a,402 b, and 402 c, one or more of the sources 402 a, 402 b or 402 c mayundergo a fast modulation operation (on the order of RF frequencies) inorder to reduce coherence artifacts for the given single source.

In another aspect of the present invention, one or more of theillumination sources 402 a-402 c may include any broadband illuminationsource known in the art. In one embodiment, one or more of theillumination sources 402 a-402 c may include, but are not limited to, ahalogen light source (HLS), as described above. In another example, oneor more of the illumination sources 402 a-402 c may include a xenon arclamp. By yet another example, one or more of the illumination sources402 a-402 c may include a deuterium arc lamp. In another embodiment, oneor more of the illumination sources 402 a-402 c may include, but is notlimited to, any discharge plasma source known in the art. In yet anotherembodiment, one or more of the illumination sources 402 a-402 c mayinclude, but are not limited to, a laser-driven plasma source. In afurther embodiment, one or more spectral filters (not shown) may bedisposed between the output of one or more broadband filters and thewavelength combiners 404 a-404 c in order to spectrally filter thespectral output of one or more broadband illumination sources.

In another aspect of the present invention, one or more of theillumination sources 402 a-402 c may include any narrowband illuminationsource known in the art. In one embodiment, one or more of theillumination sources 402 a-402 c may include, but are not limited to,one or more lasers. For instance, one or more of the illuminationsources 402 a-402 c may include, but are not limited to, one or moresemiconductor lasers. In another example, one or more of theillumination sources 402 a-402 c may include, but are not limited to, adiode-pumped solid-state laser. In another example, one or more of theillumination sources 402 a-402 c may include, but are not limited to, asuper continuum laser. In another embodiment, one or more of theillumination sources 402 a-402 c may include, but are not limited to,one or more light-emitting diodes. It should be recognized by thoseskilled in the art that the above described illumination sources do notrepresent limitations, but should merely be interpreted as illustrative.In a general sense, any illumination source capable of producingillumination in the visible, infrared, and ultraviolet spectral rangesare suitable for implementation in one or more of the illuminationsources 402 a-402 c.

It is further recognized herein the set of multiple sources 402 a-402 cmay include a combination of narrowband and broadband sources. Forexample, one or more of the sources 402 a-402 c may include a lasersource, while one or more of the remaining sources consist of abroadband lamp (e.g., laser produced plasma source) equipped with afixed or wavelength-switchable spectral filter.

FIG. 5 illustrates a multi-source illumination source 102 with intensityswitching capabilities, in accordance with an alternative embodiment ofthe present invention. In one aspect, the multi-source illuminationsource 102 of the optical metrology tool 100 includes two or more singleillumination sources, each single source having a different outputwavelength. In an additional aspect, the multi-source illuminationsource 102 of FIG. 5 includes a set of illumination switching devices502 a, 502 b, and 502 c. In this regard, the intensity contribution ofeach source 402 a, 402 b, and 402 c to the combined output beam 408 maybe controlled using the illumination switching devices 502 a, 502 b, and502 c respectively. Further, the modulation control system 112 may beconfigured to control the illumination switching devices 502 a-502 c viaillumination switching algorithm 122, thereby controlling the intensityof each wavelength component of the combined beam 408. In this manner,the modulation control system 112 may control the waveforms associatedwith each wavelength λ₁, λ₂, and up to and including λ_(N), therebytransmitting a combined waveform of selected frequency, duty cycle, andintensity of each wavelength component.

In one embodiment, the one or more of the illumination switching devices502 a, 502 b, and 502 c may include, but are not limited to, a Pockelscell disposed between a first polarizer and a second polarizer. In thisregard, the Pockels cell associated with each wavelength channel λ₁, λ₂,and λ_(N) may act as a digital ON/OFF intensity switch that isresponsive to a transmitted signal from the modulation control signal.In a further embodiment, the switching period of each Pockels cell maybe much shorter than the integration time of the detector 110, obviatingthe need for phase synchronization between the Pockels cell and thegiven source 402 a-402 c and/or detector 110.

In another embodiment, the one or more of the illumination switchingdevices 502 a, 502 b, and 502 c may include, but are not limited to, anacousto-optical switching device or, in a general sense, any fastoptical switching device known in the art.

FIG. 6 illustrates a spectral monitoring system 602 configured tomonitor one or more spectral characteristics of the modulatableillumination source 102, in accordance with one embodiment of thepresent invention. It is recognized herein that in settings where thenoise and coherence artifacts have been reduced (i.e., by reducingcoherence of illumination) accurate knowledge of the spectral propertiesof the illumination is desirable. In one embodiment, the spectralmonitoring system 602 may be used to monitor peak or centroid wavelengthfor each illumination source. It is also anticipated that the spectralmonitoring system 602 may be particularly useful in the context of drivecurrent modulated diode laser based illumination sources (describedpreviously herein) as proper monitoring of the spectral output of theillumination beam will ensure the coherence length of the givenillumination beam is reduced below an acceptable level. In this regard,one or more portions of the spectral monitor system 602 may be disposedalong the illumination pathway 604 of the optical metrology tool 100. Inthis sense, the spectral monitoring system 602 may measure one or morespectral characteristics of the illumination emanating from themodulatable illumination source 102. In one embodiment, the one or morespectral characteristics may include, but are not limited to, intensityspectra over a selected wavelength range, position of one or morespectral peaks of interest (e.g., position of centroid wavelength),full-width half-maximum (FWHM) of a spectral peak of interest, and thelike.

In a further embodiment, the spectral monitoring system may becommunicatively coupled to the modulation control system 112. In thisregard, results of a spectral measurement of illumination in theillumination pathway 604 may be transmitted to the control system 112.In a further embodiment, the modulation control system 112 may store theresults of the spectral monitoring process in a memory medium for futureuse.

In one embodiment, the spectral monitoring system 602 may monitor one ormore spectral characteristic of illumination from the illuminationsource 102 in real-time or near-real-time. For example, the spectralmonitoring system 602 may include a spectrometer suitable for real-timemeasurement of one or more spectral characteristics of illumination fromthe illumination source 102. For instance, the spectral monitoringsystem 602 may include, but is not limited to, a grating-basedspectrometer. Applicants note that a grating-based spectrometer may beparticularly useful in measuring the spectral characteristics (e.g.,centroid wavelength) of light sources used for optical metrology toolsof this invention.

In another embodiment, the spectral monitoring system 602 may monitorone or more spectral characteristics of illumination from theillumination source 102 for calibration purposes. For example, thespectral monitoring system 602 may measure one or more spectralcharacteristics of illumination from the illumination source in a toolset-up calibration process. For instance, the spectral monitoring system602 may measure one or more spectral characteristics of illuminationfrom the illumination source 102 in a tool set-up calibration process,whereby an optical metrology measurement is carried out on a calibrationtarget (i.e., target having known parameters (e.g., known CD, known thinfilm thickness and/or composition, known overlay, and the like)).Utilizing the results of the metrology measurement (e.g., thicknessmeasurement) and the results of the measured spectral characteristics ofthe illumination, the control system 112 may carry out a spectralmonitoring calibration algorithm 119 stored in the carrier medium 114.The modulation control system 112 may periodically calibrate, or“re-calculate,” one or more spectral properties of the illumination fromthe illumination source 102 based on the measurement of the calibrationsample and the measured spectral properties of the illumination. It isfurther noted that the frequency of spectral calibration may depend onthe spectral stability of the given illumination source.

In one embodiment, the calibration sample may consist of a sample havinga known thin film thickness. For example, the calibration sample mayinclude, but is not limited to, a sample having a known oxide layerthickness (e.g., a silicon-based W-chip having a known oxide thickness).In this regard, the thickness of the calibration sample may becalibrated during the calibration process carried out by the controlsystem 112. Then, the spectral characteristics of the calibration samplemay be periodically monitored using each data channel (e.g., allwavelengths of illumination, polarization states, and the like) of thesystem 100. Based on the monitoring by the spectral monitoring system602 the control system 112 may re-calculate the spectral properties(e.g., each wavelength value of the spectrum) of the illumination source102.

In an additional aspect, the modulation control system 112 may input theresults from a measurement of one or more spectral characteristics of agiven sample into the sample modeling software of the control system112. In this regard, the sample modeling software executed by thecontrol system 112 acts to correlate measured data from the sample witha given optical model. The implemented optical model may use as an inputthe one or more spectral characteristics of the given analyzed sampleacquired by the spectral monitoring system 602.

It is noted herein that the spectral monitoring system 602 may includeany spectral monitoring/measurement device known in the art. Forexample, the spectral monitoring system 602 may include, but is notlimited to, any spectrometer known in the art (e.g., grating-basedspectrometer).

FIG. 7A illustrates a block diagram view of a light-driven plasmaillumination subsystem 700 with a modulated pumping source suitable forimplementation in the optical metrology tool 100 of the presentinvention. It is noted herein that operation of plasma sources withpumping sources (e.g., pumping lasers) driven in a constant-current modemay lead to noise levels larger than desirable for optical metrologyapplications. The present invention is directed to drive currentmodulation of a pumping laser of a plasma source in order to reducenoise levels in the output illumination of the plasma source. Inparticular, the pump control system 701 of the light-driven (e.g.,laser-driven) illumination subsystem 700 may act to reduce the noiselevel within a particular frequency bandwidth by modulating the pumpingsource 702 at frequencies greater than the detector 110 bandwidth. Inthis regard, the modulation frequency is selected such that the lasermodulation does not alias to the detected frequency range of interest.

In one aspect, the plasma-based illumination subsystem 700 of theoptical metrology tool 100 may include a modulatable pumping source 702configured to generate illumination (e.g., generate illumination of aselected wavelength) and a plasma cell 706 suitable for containing aselected gas (e.g., argon, xenon, mercury, and the like). In addition,the subsystem 700 may include a set of optics 704 (e.g., focusingoptics, shaping optics, condition optics, and the like) configured tocondition and shape the beam emanating from the pumping source 702 andfurther configured to focus the beam into the volume of gas containedwithin the bulb of the plasma cell 706. It is noted herein that beamshaping and condition elements of the subsystem 700 may be utilized tooptimize, or at least improve, the shape of the beam emanating from thepumping source 702 in order to maximize pumping efficiency (or at leastattain a selected level of pumping efficiency) in the plasma cell 706.In addition, the beam shaping optics may be utilized to optimize theshape of the plasma within the plasma cell 706. By focusing light fromthe pumping source 702 into the volume of gas contained within theplasma cell 706, energy is absorbed by the gas or plasma within the bulbof the plasma cell 706, thereby “pumping” the gas species in order togenerate or sustain a plasma.

In a further aspect, broadband illumination emitted by the plasma cell706 may then be directed to the sample 106 via the illumination optics104 of the optical metrology tool 100. Then, the collection optics 108of the metrology tool 100 may direct illumination reflected or scatteredfrom the sample 106 to the detector 110.

The generation of plasma within inert gas species is generally describedin U.S. patent application Ser. No. 11/695,348, filed on Apr. 2, 2007;U.S. Pat. No. 7,435,982, issued on Oct. 14, 2008, which are incorporatedherein by reference in their entirety. In a general sense, the subsystem700 should be interpreted to extend to any plasma based light sourceknown in the art.

FIG. 7B illustrates a schematic view of the laser-driven illuminationsubsystem 700, in accordance with one embodiment of the presentinvention. In one embodiment, the optics 704 of subsystem 700 mayinclude, but are not limited to, beam conditioning/shaping optics 717configured for conditioning/shaping the beam from the modulated pumpingsource 702. Further, the optics 704 may include a set of focusing optics716 suitable for focusing illumination from the pumping source 702 intothe volume of gas 707 contained within the bulb of the plasma cell 706.

In an additional embodiment, the subsystem 700 may include a variety ofadditional optical components. For example, the subsystem 700 mayinclude, but is not limited to, a steering mirror 718 suitable fordirecting illumination 721 from the modulated pumping source 702 towardthe plasma cell 706. In a further example, the subsystem 700 mayinclude, but is not limited to, a beam splitter/dichroic mirror 722suitable for transmitting illumination from the pumping source 702 tothe plasma cell 706 and further suitable for reflecting broadbandillumination emitted by the plasma cell 706 (and directed by the ellipse720) along an output path 724 toward a set of illumination optics 104 ofthe optical metrology tool 100 (described previously herein).

Applicant notes that the above description of the laser-drivenillumination subsystem 700 is in no way limiting and should beinterpreted merely as illustrative. It is noted herein that numerouslaser-driven plasma illumination subsystems are suitable forimplementation in the present invention.

For example, the ellipse 720 may also be configured to act as a focusingelement for illumination emanating from the pumping source 702, wherebythe ellipse 720 may act to focus illumination 721 into the volume of gas707 of the plasma cell 706. In this regard, the ellipse 720 may beconfigured to both focus laser illumination from the pumping source 702into the plasma cell 706 as well as directing broadband emissions fromthe plasma cell 706 toward the downstream illumination optics 104 of themetrology tool 100. In this embodiment, the subsystem 700 may alsoinclude a collimator (not shown) configured to collimate illuminationemanating from the pumping source 702.

By way of another example, the system 700 may be configured forseparating the illumination 721 emitted by the pumping source 702 fromthe broadband emissions 724 emitted by the plasma cell 706 without theneed for beam splitter 722. In this regard, the illumination optics 104of the optical metrology tool 100 may be configured to receive broadbandemissions 724 directly from the plasma cell 706. For instance, thepumping source NA 721 may be separated from the plasma emission NA 724,whereby the pumping source NA 721 is vertically oriented, while theplasma emissions are collected along a horizontal path.

In an additional aspect, the illumination subsystem 700 includes a pumpcontrol system 701 communicatively coupled to the modulatable pumpingsource 702, wherein the modulation control system 701 is configured tomodulate a drive current of the modulatable pumping source 702 at aselected modulation frequency in order to produce time-varyingcharacteristics within the plasma/gas volume in the plasma cell 706. Forexample, the time-varying characteristics may include, but are notlimited to, time-varying thermal distributions within the plasma/gasvolume in the plasma cell 706. In a further aspect, the pump controlsystem 701 may control the modulatable pumping source 702 via pumpcontrol algorithm 720 stored as a set of program instructions 116 incarrier medium 114.

In one embodiment, the modulatable pumping source 702 of theillumination subsystem 700 includes, but is not limited to, one or morelasers. Applicant further notes that for the purposes of clarity thevarious components of the optical metrology tool 100 residing downstreamfrom the illumination optics 104 are not depicted in FIG. 7B. Applicantnotes, however, that the various components and subsystems of theoptical metrology tool 100 as described previously herein should beinterpreted to extend to the light-driven plasma source depicted inFIGS. 7A and 7B. In addition, the light-sustained plasma source depictedin FIGS. 7A and 7B may be implemented in a reflectometer, scatterometer,ellipsometer, or spectrometer configuration as discussed previouslyherein.

It is noted herein that the frequency of modulation of the pumpingsource 702 should be sufficiently above the Nyquist frequency of thedetector electronics of the optical metrology tool 100 in order tominimize aliasing in the detector 110.

In addition, the depth of modulation must be selected such thatsignificant characteristic variation is achieved within the plasma ofthe plasma cell 706 without reducing the power density within the plasmato a level where the plasma is no longer sustainable. In a furtheraspect, the pump control system 701 may act to modulate the drivecurrent of the laser pumping source 702, thereby modulating the pumplaser intensity and wavelength. Modulation in intensity and wavelengthin the light output of the pumping source 702 may act to generateoscillating characteristics (e.g., temperature distribution) within theplasma of the plasma cell 706. Since the plasma emissions from theplasma cell 706 typically pass through numerous optical components,including one or more apertures, which limit the spatial extent of theplasma imaged through the optical system, the modulation of the spatialdistribution of the plasma source may contribute on the same order asthe modulation of the spatially integrated power collected from thelight source. Applicants have found a significant reduction in noiselevel across a wide range of modulation amplitudes for a modulationfrequency of approximately 20 kHz to 40 kHz. Applicants have also shownthat square wave and sine wave modulation of the pumping source 702 areeffective in noise level reduction. Applicants note that the frequencyrange and types of waveforms provided above are in no way limiting andare provided merely for purposes of illustration. It is anticipated thata variety of modulation waveforms and frequency ranges are within thescope of the present invention.

It is further noted herein that by controlling the plasmacharacteristics as described above and integrating over multiplemodulation periods for each detector sample the illumination subsystem700 may act to reduce the effects of randomness on the overall noiselevel of the overall optical metrology tool 100.

It is further contemplated that each of the embodiments of the methoddescribed above may include any other step(s) of any other method(s)described herein. In addition, each of the embodiments of the methoddescribed above may be performed by any of the systems described herein.

While particular aspects of the present subject matter described hereinhave been shown and described, it will be apparent to those skilled inthe art that, based upon the teachings herein, changes and modificationsmay be made without departing from the subject matter described hereinand its broader aspects and, therefore, the appended claims are toencompass within their scope all such changes and modifications as arewithin the true spirit and scope of the subject matter described herein.

Furthermore, it is to be understood that the invention is defined by theappended claims. Although particular embodiments of this invention havebeen illustrated, it is apparent that various modifications andembodiments of the invention may be made by those skilled in the artwithout departing from the scope and spirit of the foregoing disclosure.Accordingly, the scope of the invention should be limited only by theclaims appended hereto. It is believed that the present disclosure andmany of its attendant advantages will be understood by the foregoingdescription, and it will be apparent that various changes may be made inthe form, construction and arrangement of the components withoutdeparting from the disclosed subject matter or without sacrificing allof its material advantages. The form described is merely explanatory,and it is the intention of the following claims to encompass and includesuch changes.

What is claimed:
 1. An optical metrology tool, comprising: a modulatablepumping source configured to generate illumination beam; a plasma cell,the plasma cell including a bulb for containing a volume of gas; a setof optical elements configured to shape the illumination beam and focusthe illumination beam from the modulatable pumping source into thevolume of gas in order to maintain a plasma within the volume of gas; aset of illumination optics configured to direct the illumination beamfrom the plasma cell to the surface of a sample; a set of collectionoptics; a detector configured to detect at least a portion ofillumination emanating from a surface of a sample, wherein the set ofcollection optics is configured to direct illumination from the surfaceof the sample to the detector; a pump control system communicativelycoupled to the modulatable pumping source, wherein the pump controlsystem is configured to modulate a drive current of the modulatablepumping source at a selected modulation frequency in order to producetime-varying characteristics within the plasma contained within theplasma cell.
 2. The optical metrology tool of claim 1, wherein themodulatable pumping illumination source comprises: one or more lasers.3. The optical metrology tool of claim 1, wherein the time-varyingcharacteristics comprise: time-varying thermal distributions.
 4. Theoptical metrology tool of claim 1, wherein the detector comprises: atleast one of a CCD array, a CMOS array, one-dimensional photodiodearray, a two-dimensional photodiode array.
 5. The optical metrology toolof claim 1, wherein the detector is synchronized with the controlsystem.
 6. The optical metrology tool of claim 1, wherein the pumpcontrol system reduces the noise level within a particular frequencybandwidth by modulating the pumping source at frequencies greater than abandwidth of the detector.
 7. The optical metrology tool of claim 1,wherein the plasma cell is configured to contain at least one of argon,xenon, or mercury.
 8. The optical metrology tool of claim 1, wherein thesample comprises a semiconductor wafer.
 9. The optical metrology tool ofclaim 1, further comprising a dichroic mirror configured to transmitillumination from the pumping source to the plasma cell and furtherconfigured to reflect broadband illumination emitted by the plasma cell.10. The optical metrology tool of claim 1, wherein the optical metrologytool is arranged as at least one of a reflectometer, scatterometer,ellipsometer, or spectrometer.
 11. The optical metrology tool of claim1, wherein the modulatable pumping source is modulated at a frequencybetween 20 kHz and 40 kHz.
 12. The optical metrology tool of claim 1,wherein the modulatable pumping source is configured to modulate atleast one an intensity or wavelength of illumination beam.